1. Field of the Invention
Broadly speaking, this invention relates to electrometers. More particularly, in a preferred embodiment, this invention relates to a range switching circuit which permits the construction of an electrometer having only non-critical, external circuit connections.
2. Discussion of the Prior Art
Solid state electrometers which employ field-effect-transistors to attain a high input impedance have proved uniquely suitable for the measurement of extremely low currents; for example, currents in the picoamp range such as emanate from ionization chambers which are exposed to ionizng radiation. A high degree of accuracy and stability may be attained with such electrometers, for example, by the use of negative feedback obtained from high value resistors inserted into the feedback loop. Such an arrangement, which is known in the prior art, makes the signal stability primarily a function of the stability of the passive components in the feedback loop (e.g., the high value resistors). By the choice of suitable resistors, the temperature stability over the range of interest (say, -40.degree. C. to +60.degree. C.) may be held to within .+-.10% of room temperature which is generally considered acceptable. Also, with this arrangement the electrometer performance is predictable and highly reproducible.
One of the problems affecting feedback-type electrometers is the need to provide range switching to cover several decades of input signal (i.e., radiation dose rates). In commercial, laboratory-type, electrometers it is customary to use a special, high-impedance range switch, for example, a teflon-insulated or ceramic switch, which, in conjunction with a plurality of high value resistors connected to the range switch, permits the electrometer to measure multiple ranges of input current. However, for use in certain critical environments, or where a compact instrument is needed, the use of such a range switch is undesirable; furthermore such switches are not readily available at low cost. Also, the design of a ruggedized ion-chamber module containing all the high-impedance components is compromised when external pin connections must be provided for the range resistors, due to potentially troublesome external leakage paths. Put another way, the external contamination of the ion chamber module in a field environment which is possible with such an arrangement can severely degrade the instrument's performance. It is, thus, an object of this invention to provide a range switching arrangement which circumvents the aforementioned problems, thereby permitting construction of an ionization-chamber module having only non-critical, external circuit connections. Another object of this invention is to provide a switching arrangement which is compatible with battery-powered, portable equipment, i.e., a switching arrangement that consumes minimal power.